This invention relates to the use of certain thiazolones to enhance myocardial contractile force. These compounds are useful as cardiotonics in the treatment of heart failure.
Heart failure is that physiological condition resulting from the inability of the ventricular myocardium to maintain adequate blood flow to the peripheral body tissues and includes congestive heart failure, backward and forward heart failure, right ventricular and left ventricular heart failure, and low-output heart failure. Heart failure can be caused by myocardial ischemia, myocardial infarction, excessive alcohol usage, pulmonary embolism, infection, anemia, arrhythmias, and systemic hypertension. Symptoms include tachycardia, fatigue with exertion, dyspnea, orthopnea and pulmonary edema.
Treatment involves either removal or correction of the underlying cause or involves control of the heart failure state. Management or control can be accomplished by increasing cardiac output or by decreasing cardiac workload. While workload can be reduced by reduction of physical activities and physical and emotional rest, increasing cardiac output has traditionally involved therapy with digitalis or a digitalis glycoside and more recently vasodilator therapy. Digitalis stimulates contractile force of the heart which increases cardiac output and improves ventricular emptying. In this way digitalis therapy normalizes venous pressure and reduces peripheral vasoconstriction, circulatory congestion and organ hypoperfusion
Unfortunately, optimal doses of digitalis vary with the patient's age, size and condition and the therapeutic to toxic ratio is quite narrow. In most patients the lethal dose is only about five to ten times the minimal effective dose with toxic effects becoming apparent at only 1.5 to 2.0 times the efective dose. For these reasons, dose must be carefully tailored to suit the individual and frequent clinical examination and electrocardiogram are necessary to detect early signs of digitalis intoxication. Despite this care digitalis intoxication is reported in up to one-fifth of hospitalized patients undergoing therapy.
Vasodilator therapy increases cardiac output and improves ventricular emptying by reducing the systemic blood pressure against which the heart must pump. However, in severe heart failure a vasodilator alone may not improve cardiac function sufficiently due to the weakness of the myocardial contractility necessitating the concomitant use of digitalis. Moreover, a rapid tolerance has been reported to develop to the effects of vasodilator therapy in heart patients. The need for less toxic and more effective cardiotonic agents is readily apparent. Applicants have discovered certain phenyl thiazolones which possess potent cardiotonic activity and by comparison to digitalis have few toxic effects.